A stacked chip semiconductor device known as a conventional semiconductor device is manufactured by stacking a plurality of semiconductor chips to be mounted on a substrate in order to improve the packaging density of the semiconductor chips and further encapsulating them with a molded resin. This type of semiconductor device is named also as a stacked package (see, for example, JP H11 (1999)-204720 A, page 7 and FIG. 3 thereof).
FIGS. 5A and 5B show a conventional stacked chip semiconductor device. FIG. 5A is a cross-sectional view and FIG. 5B is a perspective plan view of the same semiconductor device.
In this stacked chip semiconductor device, a first semiconductor chip 103 is mounted on a substrate 101 via a first adhesive layer 102 by a so-called flip-chip-packaging. The substrate 101 has electrode pads 101a on its upper face and a land 101b on its bottom face. The first semiconductor chip 103 having bumps 103a is mounted with the bumps 103a facing downward.
On an upper part of the first semiconductor chip 103, a second semiconductor chip 105 having electrode pads 105a is mounted via a second adhesive layer 104. The electrode pads 105a of the second semiconductor chip 105 and the electrode pads 101a of the substrate 101 are electrically connected to each other via wires 106. The first semiconductor chip 103, the second semiconductor chip 105 and the wires 106 are encapsulated with a molded resin 107.
Here, a central axis A as a point of symmetry for the planar shape of the substrate 101, a central axis B of the first semiconductor chip 103, and a central axis C of the second semiconductor chip 105 are disposed to coincide with each other.
The first adhesive layer 102 is a cured liquid adhesive. After mounting the first semiconductor chip 103 on the substrate 101, the adhesive is charged into an entire region of a gap between the first semiconductor chip 103 and the substrate 101, excepting the bumps 103a, and cured in the gap. Thereby, the first semiconductor chip 103 is fixed firmly to the substrate 101 so that stress to be applied to the bumps 103a will be distributed to the entire region of the substrate 101 for mounting the chip, and the reliability of the semiconductor device is improved.
In the conventional stacked chip semiconductor device, as shown in FIGS. 5A and 5B, the first adhesive layer 102 spreading out of the periphery of the first semiconductor chip 103 forms a fringe 102a (hereinafter, referred to as ‘fillet’). To accommodate formation of the fillet 102a, the substrate 101 must be enlarged by the area of the fillet 102a. This has been a barrier for the miniaturization of the semiconductor device.
This problem will be described further below. As mentioned above, the first adhesive layer 102 is formed by curing a liquid adhesive, and the surface is shaped with a certain curvature due to the surface tension of the adhesive. Especially, since the adhesive forms a pool at the side among the four sides of the periphery of the first semiconductor chip 103 from which the adhesive is injected, the fillet 102a will have a large volume and occupy more surface area of the substrate 101. The electrode pads 101a for electrically connecting the second semiconductor chip 105 and the substrate 101 are arranged around the region of the substrate 101 on which the fillet 102a is to be formed. When the adhesive composing the fillet 102a adheres to the electrode pads 101a, a connection failure of the wires 106 may occur and degrade the reliability of the semiconductor device. Therefore, the electrode pads 101a must be disposed keeping a predetermined distance from the end of the region on which the fillet 102 is formed, and this has been a barrier in reducing the area of the substrate 101.
The fillet 102a will be smaller at the three sides of the periphery of the first semiconductor chip 103 other than the side for adhesive injection. For this reason, it is possible to design the substrate 101 so that the electrode pads 101a at the three sides irrelevant to injection of the adhesive are disposed closer to the first semiconductor chip 103 while the electrode pads 101a are distant from the first semiconductor chip 103 at the side for adhesive injection. In such a case, however, the wires 106 at the side for adhesive injection become longer than the wires 106 at any of the three remaining sides. When the length of the wires at the four sides is not uniform, facility conditions for deciding the loop shapes of the wires should be modified for every side, and this may increase of the working times and decrease the yield.
The fillet 102a may be formed to spread over the first semiconductor chip 103. When this reaches the region for forming the second adhesive layer 104, the second semiconductor chip 105 cannot be fixed firmly to the first semiconductor chip 103, thereby reducing the reliability of the stacked chip semiconductor device. The fillet 102a tends to spread over the semiconductor chip 103 particularly at the side for adhesive injection. And the adhesion failure of the second semiconductor chip 105 to the upper part of the first semiconductor chip 103 will occur easily when the distance from the side for adhesive injection to the periphery of the second semiconductor chip 105 is short.